Heavy slabs and sheets of material are usually stored and stacked upright standing or in slightly inclined orientation. Handling of this type of materials often entails use of lifting clamps that grip the sheet (hereinafter used generically to also encompass slabs and other planar objects) at its upper edge for hoisting. Consequently, it is convenient in the following description of known devices and the lifting device in accordance with the present invention to use reference terminology such as ‘vertical’, ‘horizontal’, ‘upper’, ‘lower’ and similar when describing operation and components of such clamping devices, bearing in mind that these devices may also be used in a ‘horizontal’ or other orientation, eg as a simple clamp or a haulage attachment. Thus, unless otherwise clear in the context, such reference terms are not to be interpreted as a limitation.
A lifting clamp of the type with which the present invention is concerned is known from U.S. Pat. No. 5,893,595 (Corbett). The lifting device includes a rigid frame comprising vertical, parallel spaced apart side plates, the upper ends of which are rigidly secured together by four tubular cross-members. The lower portion of one of the side plates is angled away in downward orientation from the other side plate, the latter providing a fixed clamping jaw of the device. A vertically extending plate is mounted for horizontal sliding movement on the cross-members between the side plates and provides a movable jaw of the device. An actuator carriage which is disposed for guided vertical up and down movement is located between the movable jaw and the lower, angled portion of the frame side plate, whereby a set of rollers of the carriage respectively engage the facing surfaces of the movable jaw plate and the angled portion of the frame side plate. The carriage is connected to a strip member that extends beyond the upper end of the frame and has a lifting lug to which is attachable a lifting cable or chain. In order to lift (or otherwise handle) sheet material, the device is placed over the upper edge of the sheet so that it is received between the fixed and movable jaw plates, the carriage is raised by lifting the strip member through pulling the lifting cable upwards, whereby the carriage travels on the angled frame side plate portion and displaces the movable jaw horizontally until it abuts on the facing surface of the sheet material. Upon increasing the upward pulling force, the sheet material is frictionally clamped sufficiently tightly for it to be lifted with the device. In essence, clamping of the sheet material between the plate jaws is achieved by wedging the carriage between the frame side plate and the movable jaw, and the clamping force is maintained for as long as there is upward force being on the lifting cable.
A lifting clamp of similar design to that of Corbett is disclosed in German patent publication DE 199 23 788 A1 (Scheibenbogen GmbH & Co KG), where, however, the mechanism employed to move the movable jaw into gripping engagement with the sheet material comprises a cam pulley instead of a wedging carriage. The pulley is mounted on the movable jaw plate with its axis of rotation perpendicular to the plane of the movable jaw plate, four rollers serving to support the pulley in parallel relationship at the facing jaw surface. Two cams are arranged on the opposite face of the pulley, concentrically with and symmetrical about the rotation axis. The cams extend equiradially over a sector of the of the pulley circumference. The height of the cams increase from near the pulley face to a maximum height that is dictated by the maximum spacing between the fixed and movable jaws of the clamp in their fully spaced apart position. The cams thus provide sloping guide and bearing surfaces for respective actuator rollers that are secured in fixed relationship on the frame plate of the device that faces the pulley. An actuator cable that is secured to the pulley perimeter is used to rotate the pulley, the actuator cable advantageously terminating in a hoop on which a hoisting cable or chain may be attached. In operation of the clamp, when a pulling force is exerted on the actuator cable, rotation of the pulley causes the actuator rollers to travel along the inclined (or curved) bearing surfaces of the cams, thereby displacing the movable jaw away from the facing frame plate towards the fixed jaw and clamping sheet material received between the jaws.
One disadvantage that has been observed with lifting clamps made in accordance with Corbett as well as Scheibenbogen is the tendency of the clamp frame, on lifting of in particular thin sheet material from the ground, of rotating into a different spatial attitude from the initially given “no load” attitude, the later being characterised by the substantially vertical orientation of the clamping surfaces of the jaws when these are clamped onto a sheet of material resting upright on the ground. This rotation induces swaying and swinging of the sheet material at the lifting cable that makes precise hoisting difficult, and increases breakage risk upon hitting against objects in the vicinity of the sheet. The moment that causes this rotation is induced by the presence of an out-of-alignment force pair on initial lifting off of the sheet attached to the clamp frame (upward directed lifting force vs downward directed weight of sheet material and clamp). This movement inducing moment decreases and ceases as the clamp frame and the sheet material rotate into and eventually assume a final, slightly inclined orientation with respect to the vertical, as the respective centres of gravity of the clamp and the sheet material seek to and ultimately align themselves along the same (vertical) line along which the lifting force is being exerted.
This problem stems from the lay-out of the clamp as such, ie due to the presence of a fixed clamp against which a single movable clamping jaw plate is forced. With such lay out, the line along which gravity force acts on the upright sheet material locates within the clamp frame at different traverse locations, depending on the thickness of the sheet, and does not align with the point at which the upward directed hoisting force acts on the clamp frame, notwithstanding the lifting force ultimately acts on the carriage or cam pulley of the clamp actuating mechanism in close vicinity of the secured sheet material.
Equally, whilst the cam pulley clamping mechanism of Scheibenbogen has the advantage over Corbett's of providing a more even distribution of clamping pressure onto the sheet material, the Scheibenbogen lifting clamp has a more pronounced tendency of swaying under load, which results in difficulties in controlling movement of sheet material during transport.
One object of the present invention is to provide a lifting device of the aforementioned type in which the tendency of the clamp of swaying into an inclined position during lifting of sheet material is minimised.